Apodization for Pupil Imaging Scatterometry

ABSTRACT

The disclosure is directed to various apodization schemes for pupil imaging scatterometry. In some embodiments, the system includes an apodizer disposed within a pupil plane of the illumination path. In some embodiments, the system further includes an illumination scanner configured to scan a surface of the sample with at least a portion of apodized illumination. In some embodiments, the system includes an apodized pupil configured to provide a quadrupole illumination function. In some embodiments, the system further includes an apodized collection field stop. The various embodiments described herein may be combined to achieve certain advantages.

PRIORITY

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/730,383, entitled APODIZATION FOR PUPIL IMAGINGSCATTEROMETRY METROLOGY, By Andy Hill et al., filed Nov. 27, 2012, whichis currently co-pending, or is an application of which currentlyco-pending application(s) are entitled to the benefit of the filingdate. The foregoing provisional application is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to the field of opticalmetrology and more particularly to apodization for an optical metrologysystem.

BACKGROUND

Optical metrology is often utilized to measure optical and/or structuralcharacteristics of either device or test features during semiconductormanufacture. For example, optical or structural characteristics mayinclude critical dimensions such as height, side wall angle, pitch,linewidth, film thickness, refractive indices, and overlay betweendifferent layers or between exposures within a single layer. Apodizationmay be implemented in optical metrology systems to control angular andspatial distribution of illumination at well-defined locations along theoptical path. Apodization is particularly important when metrologyaccuracy and precision depends on ability to retrieve high fidelityspectroscopic or angular information from small metrology targets. Insuch cases, there is a need to prevent signal contamination resultingfrom either unwanted scattering from areas outside a designatedmetrology target on sample or due to scattering from intermediateoptical components or apertures along the optical path.

In the case of an angle resolved (pupil imaging) scatterometer, a knownpractice in the art is the combination of: (1) a simple flat topaperture (pupil) stop in the illumination path which restricts theillumination numerical aperture (NA) so that different diffractionorders from the metrology target can be isolated in the collectionpupil; and (2) a simple flat top field stop in the illumination path tolocalize illumination on a small target. With the foregoingarchitecture, the illumination field stop becomes the limiting apertureof the pupil imaging system. The hard edges of the illumination fieldstop cause ringing in the images of the illumination aperture stop, andthe ringing results in interaction or interference between orders (e.g.0th order and 1st order) in the pupil image. One method of resolvingthis problem is field apodization in the illumination path of theoptical metrology system. With field apodization, the introduction of asmoothly varying transmission function in the field results in asmoothly varying and rapidly decaying function in the conjugate pupilplane, effectively suppressing the ringing which results in interferencebetween orders.

The foregoing approach may be appropriate for a spatially incoherentsystem with illumination etendue to spare. However, for opticalmetrology systems employing coherent illumination sources, such as alaser-based system with high spatial and temporal coherence, substantialnoise issues emerge. Shaping the spatially coherent illumination beam sothat it has low tails in the field plane is desirable in order tominimize periphery contamination and diffraction by the edge of thetarget. Low tails in the pupil plane are desirable to minimizeinteraction and interference between diffraction orders and clipping bythe objective pupil. The beam can potentially be shaped by somecombination amplitude and phase apodization in the illumination fieldstop or the illumination aperture stop. To average out the effects oftarget noise, it is desirable to scan the spatially coherentillumination spot over the target during a measurement. If beam shapingis performed at the illumination field plane, and the spot scanningmechanism is situated before this field plane, then the beam shape inthe field and pupil planes will change as the spot scans across thefield apodizer. This introduces fluctuations in the overall beamintensity as well as asymmetries in the distribution of light in thepupil.

SUMMARY

The present disclosure is directed to curing some or all of theforegoing deficiencies in the art utilizing one or more of theapodization schemes described below.

Various embodiments of the disclosure are directed to a system forperforming optical metrology including at least one illumination sourceconfigured to illuminate the sample and at least one detector configuredto receive at least a portion of illumination scattered, reflected, orradiated from the sample. At least one computing system communicativelycoupled to the one or more detectors may be configured to determine atleast one spatial attribute of the sample, such as an optical orstructural characteristic, based upon the detected portion ofillumination.

The system may include an apodizer or an apodized pupil disposed withina pupil plane of the illumination path. The apodizer may be configuredto apodize illumination directed along the illumination path. In someembodiments, the system may further include an illumination scannerdisposed along the illumination path and configured to scan a surface ofthe sample with at least a portion of the apodized illumination. Thesystem may further include an illumination field stop configured toblock a portion of illumination directed along the illumination pathfrom scanning the surface of the sample and a collection field stopconfigured to block a portion of illumination directed along thecollection path from being received by the detector.

In some embodiments, the system may include an apodized pupil disposedalong the illumination path. The apodized pupil may include at leastfour elongated apertures configured to provide a quadrupole illuminationfunction. The apodized pupil may be further configured to apodizeillumination directed along the illumination path. Further, anillumination field stop disposed along the illumination path may beconfigured to block a portion of illumination directed along theillumination path from impinging upon or scanning the surface of thesample.

In some embodiments, the system may include an apodizer disposed withina pupil plane of the illumination path and may further include anapodized collection field stop disposed along the collection path. Theapodized collection field stop may be configured to apodize illuminationdirected along the collection path and further configured to block aportion of illumination directed along the collection path from beingreceived by the detector.

Those skilled in the art will appreciate that the foregoing embodimentsand further embodiments described below may be combined to achievevarious advantageous. Accordingly, the configurations described hereinshould not be construed as limitations of the disclosure unlessotherwise noted. Further, it is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory only and are not necessarily restrictive of the presentdisclosure. The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate subject matter of thedisclosure. Together, the descriptions and the drawings serve to explainthe principles of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 is a block diagram illustrating an optical metrology systemincluding an apodizer disposed within a pupil plane of the system, inaccordance with an embodiment of this disclosure;

FIG. 2 is a block diagram illustrating the optical metrology systemfurther including an illumination scanner disposed within the pupilplane of the system, in accordance with an embodiment of thisdisclosure;

FIG. 3 is a block diagram illustrating the optical metrology system,wherein the illumination scanner is disposed after an illumination fieldstop, in accordance with an embodiment of this disclosure;

FIG. 4 is a block diagram illustrating the optical metrology systemincluding an apodized illumination field stop, in accordance with anembodiment of this disclosure;

FIG. 5 is a block diagram illustrating the optical metrology system,wherein the apodizer is disposed after the illumination field stop andthe illumination scanner, in accordance with an embodiment of thisdisclosure;

FIG. 6 is a block diagram illustrating the optical metrology system,wherein the apodizer and the illumination field stop are disposed afterthe illumination scanner, in accordance with an embodiment of thisdisclosure;

FIG. 7 is a block diagram illustrating the optical metrology systemfurther including an apodized collection field stop, in accordance withan embodiment of this disclosure;

FIG. 8 is a block diagram illustrating the optical metrology system,wherein the illumination scanner is configured to scan illuminationalong a sample surface and further configured to descan illuminationcollected from the sample surface, in accordance with an embodiment ofthis disclosure;

FIG. 9A is a graphical plot of exemplary apodizer scalar pupil planeprofiles, in accordance with an embodiment of this disclosure;

FIG. 9B is a graphical plot of wafer plane scalar intensity (logarithmicscale) for the exemplary apodizer scalar pupil plane profiles, inaccordance with an embodiment of this disclosure;

FIG. 9C is a graphical plot of resultant scalar profiles at pupil imagesensor (i.e. detector) plane for the exemplary apodizer scalar pupilplane profiles, in accordance with an embodiment of this disclosure;

FIG. 10A illustrates a pupil configured for a quadrupole illuminationfunction, in accordance with an embodiment of this disclosure; and

FIG. 10B illustrates a plurality of pupil configurations, in accordancewith an embodiment of this disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

FIGS. 1 through 10B generally illustrate apodization schemes for anoptical metrology system 100, such as an angle resolved (pupil imaging)scatterometer or the like. According to various embodiments, apodizationat the pupil and/or detector plane enables high performance metrology ontargets with relatively small dimensions. Further, the embodiments thatfollow include configurations associated with certain advantages inmeasurement quality, performance, and/or precision. It is noted that theembodiments described below serve an illustrative purpose and thatvarious portions of multiple embodiments may be combined to arrive atfurther embodiments having selected advantages.

In general, the following embodiments are directed to one or more of thefollowing advantages. The embodiments of system 100 may includeconfigurations for shaping illumination directed along an illuminationpath to avoid illuminating outside of a target region of a metrologysample 102, such as a semiconductor wafer or mask. Further, theembodiments of system 100 may include configurations for mitigating orexcluding illumination diffracted or scattered from optical surfacesand/or aperture edges along the illumination path to avoid contaminatingpupil regions. The embodiments of system 100 may further includeconfigurations for shaping or excluding stray illumination reflected,scattered, or radiated along a collection path to avoid detection ofillumination from regions outside of the target region of the sample102. For instance, some configurations may be directed to blocking orexcluding portions of illumination diffracted from an objective lens ora collection field stop. Further goals or advantages are discussed belowwith respect to the following embodiments of system 100.

As shown in FIG. 1, the system 100 may include at least one spatiallycoherent (e.g. laser) or incoherent (e.g. laser sustained plasma “LSP”or laser driven light source “LDLS”) illumination source 104. Several ofthe embodiments herein are directed to an optical metrology systemsbased on spatially coherent illumination sources; however, many of theadvantages provided by the discussed configurations are applicable tosystems based on spatially incoherent illumination sources as well. Insome embodiments, the illumination source 104 may be configured toprovide illumination along an optical fiber 106 leading to a free spaceillumination path.

In some embodiments, the system 100 includes an apodizer 108, such asstandalone apodization element or an apodized pupil, disposed within thepupil plane of the illumination path. The system 100 may further includean illumination field stop 112 disposed along the illumination path. Theillumination field stop 112 may be configured to block a portion of theillumination directed along the illumination path to localizeillumination at a targeted region of the sample 102 and filter parasitic(scattered or diffracted) illumination from upstream components.

In some embodiments, the system 100 further includes an illuminationscanner 110, such as a scanning mirror, disposed between the apodizer108 and the illumination field stop 112. For example, the illuminationoptics may be arranged such that illumination from the illuminationsource 104 is directed through the apodizer 108 and then scanned acrossthe illumination field stop 112 by the illumination scanner 110.Alternatively, as shown in FIG. 5, the illumination optics may bearranged such that illumination from the illumination source 104 isdirected through the illumination field stop 112 and then scanned acrossthe apodizer 108 by the illumination scanner 110. The illuminationscanner 110 may include or may be coupled to one or more actuatorsenabling the illumination scanner 110 to spot scan a targeted region ofsample 102 with apodized illumination that is further shaped accordingthe field stop 112. The illumination optics may be further arranged suchthe illumination scanner 110 is disposed within the pupil plane. Forexample, as shown in FIG. 2, the illumination scanner 110 may beconjugate to the pupil plane. Placing the illumination scanner 110 atthe pupil plane may improve stability of an apodization functionprovided by the apodizer 108 during spot scan of at least a portion ofthe apodized illumination at the targeted region of the sample 102.

In some embodiments, as illustrated in FIG. 3, the apodizer 108 and theillumination field stop 112 are disposed before the illumination scanner110. As such, illumination received by the illumination scanner 110 isapodized and further shaped according to the illumination field stop112. This arrangement allows the illumination field stop 112 to includea smaller aperture because illumination does not need to be scannedacross the field stop 112. The illumination field stop 112 can,therefore, filter off more of the spatial noise caused by the apodizer108 and the optical fiber 106. Further, there is less chance ofintroducing intensity noise caused by a time dependency in diffractionof spot off edges of the field stop 112. When the illumination scanner110 is disposed after the apodizer 108 and the illumination field stop112, strong diffractions due to scan of field stop edges may be avoided.

FIG. 4 illustrates a further embodiment where the illumination fieldstop 112 may also be apodized to introduce field apodization in additionto pupil apodization provided by the pupil apodizer 108. The apodizedillumination field stop 112 may allow for improved ability to shapeillumination directed along the illumination path. Intensity modulationand changes to pupil distribution may occur during spot scan; however,diffraction effects due to the spot reaching edges of the illuminationfield stop 112 may be significantly mitigated by the apodization.Mitigation of the diffraction effects is important because, if notcontrolled, mixing of pupil points may occur. Since the image at theillumination field stop 112 is ultimately imaged to the sample 102,field apodization may further reduce spot intensity at the edges of thetargeted region of the sample 102 and at edges of a collection fieldstop 120, thereby suppressing mixture of pupil points at the collectionpupil.

As discussed above, FIG. 5 illustrates an embodiment where theillumination optics may be arranged such that illumination from theillumination source 104 is directed through the illumination field stop112 and then scanned across the apodizer 108 by the illumination scanner110. This arrangement may allow for a relatively small illuminationfield stop 112 disposed before the illumination scanner 110 to mitigateincoming noise from the source 104 and/or optical fiber 106. Further,locating the apodizer 108 after the illumination scanner 110 may allowfor enhanced ability to shape illumination scanned across the sample102.

Alternatively, as shown in FIG. 6, the apodizer 108 and the illuminationfield stop 112 may be disposed after the illumination scanner 110.Disposing the apodizer 108 after the illumination scanner 110 may allowfor a stationary apodization function in the illumination pupil becausethe illumination scanner 110 only affects the angle of illuminationpassing through the apodizer 108. Further, the illumination field stop112 disposed after the apodizer 108 may be enabled to filter outparasitic illumination from upstream components including the apodizer108, illumination scanner 110, optical fiber 106, and any additionalillumination optics (e.g. various lenses).

The system 100 may further include a beam splitter 114 configured todirect illumination from the illumination path through an objective lens116 to illuminate the sample 102. The system 100 may include a stage 118configured to support the sample 102. In some embodiments, the stage 118may further include or may be coupled to at least one actuator. Theactuator may be configured to translate or rotate the stage 118 todispose the sample 102 at a selected position. Accordingly, illuminationmay be targeted or scanned at a selected region of the sample 102 viaactuation of the sample stage 118. Alternatively or in addition, one ormore of the illumination optics, such as the objective 116 may beactuated to target a selected region of the sample 102 and/or adjustfocus of illumination targeted at the sample 102.

Illumination may be scattered, reflected, or radiated by the targetedregion of the sample 102. The system 100 may include at least onedetector 122, such as a camera, a spectrometer, a photodiode, or anyother photodetector which is configured to receive at least a portion ofthe scattered, reflected, or radiated illumination from the sample 102.In some embodiments, a collection field stop 120 is configured to blockat least a portion of illumination directed from the sample 102 along acollection path leading to the detector 122 to filter out parasiticillumination, such as illumination diffracted or scattered by the beamsplitter 114, objective lens 116 and/or any other collection optics.

In some embodiments, the collection field stop 120 may further beapodized, as shown in FIG. 7. It is noted that the inclusion of theapodized collection field stop 120, among other features, may besupported by any of the embodiments described herein. The apodizedcollection field stop 120 may advantageously reduce sensitivity todecentering errors of beam position with respect to the center of thetargeted region of the sample 102 resulting from diffraction orscattering from edges of the collection field stop 120. In particular, acollection field stop with small numerical aperture (NA) may be moresusceptible to decentering errors and may, therefore, benefit greatlyfrom apodization. For further explanation, sensitivity to decenteringmay result from interference between wanted diffraction from a givenorder (e.g. 1^(st) order diffraction) and unwanted diffraction fromanother order (e.g. 0^(th) order diffraction) scattered from the fieldstop 120. Interference between diffraction orders can be suppressed bycollection field stop apodization by shaping illumination to compensatefor the diffraction or scattering effects. Further, apodization of thecollection field stop 120 may reduce diffraction or scattering effectsfrom edges of pupil apertures in pupil planes located after thecollection field stop 120. Suppressing parasitic (diffracted orscattered) illumination from reaching the detector 122 may allow forimproved metrology performance by reducing inaccuracy caused bydecentering errors and allowing for greater precision.

The system 100 may include at least one computing system 124communicatively coupled to the one or more detectors 122. The computingsystem 124 may be configured to determine at least one spatial attributeof the sample 102 based upon the detected portion of illuminationscattered, reflected, or radiated from the targeted region of the sample102. For example, the computing system 124 may be configured todetermine an optical or structural characteristic of the sample 102 ordefect information associated with the sample 102 according to one ormore of the metrology and/or inspection algorithms known to the art. Thecomputing system 124 may be configured to execute at least one metrologyor inspection algorithm embedded in program instructions 128 stored byat least one communicatively coupled carrier medium 126. In someembodiments, the computing system 124 includes at least one single-coreor multiple-core processor configured to execute the programinstructions 128 from the communicatively coupled carrier medium 126.Further, it should be recognized that any of the various steps orfunctions described throughout the present disclosure may be carried outby a single computing system or by multiple computing systems.

FIG. 8 illustrates another embodiment of the system 100 where theillumination scanner 110 is disposed along a portion of the illuminationpath and a portion of the collection path. Accordingly, the illuminationscanner 110 may be configured to spot scan the targeted region of thesample 102 with illumination transferred along the illumination path,and further configured to descan illumination scattered, reflected, orradiated from the sample 102 along the collection path to the detector122. By scanning and descanning the illumination, respectively, targetedto and collected from the sample 102, the illumination scanner 110 mayreduce decentering error and improve uniformity of illumination receivedat the detector 122. Hence, the scanning/descanning optical arrangementmay improve measurement performance.

The use of apodization in pupil imaging scatterometers is described inpart by US Pub. No. 20080037134, incorporated by reference herein. Theapodizer 108 and, in various embodiments, the apodized illuminationfield stop 112 and/or the apodized collection field stop 120 mayincorporate any of the apodization technologies discussed or referencedby US Pub. No. 20080037134. One of the key characteristics of anapodizer is its transmission profile as a function of radial dimension.Apodization functions are often trapezium or Gaussian in form. In someembodiments of the system 100, apodization profiles may further include,but are not limited to, top hat, optimized top hat, Gaussian, hyperbolictangent, or Blackman forms. Rather than a polar form apodizationprofile, the 2D apodization distribution can also be implemented inCartesian form by multiplying a 1D apodization distribution for the Xdirection by the corresponding one for the Y direction. As furtherdiscussed below, an apodization profile may be selected according to acost functional to improve or optimize system performance.

FIGS. 9A through 9C illustrate examples of the scalar distribution for anumber of different apodization profiles and the corresponding resultantprofiles at the sample imaging plane and the collection pupil plane. Forexample, FIG. 9A shows scalar pupil plane profiles (transmission vs.pupil coordinates) exemplary of top hat, optimized top hat, Gaussian,and hyperbolic tangent profiles. FIG. 9B shows scalar intensity at thesample imaging plane corresponding to the exemplary apodizationprofiles, and FIG. 9C shows the corresponding resultant scalar profilesat the detector (i.e. collection) pupil plane. As illustrated by theexemplary plots in FIGS. 9A through 9C, there may be a significantreduction of intensity in the periphery of the illumination spot atsample coordinates, thus leading to reduced signal contamination at thedetector 122 from regions outside the targeted region of the sample 102.

In some embodiments, the apodization profile may be selected accordingto a cost functional. For example, the apodization profile may bespecified for substantially maximizing tail to peak ratio at a givenlocation in the aperture on the pupil detector 122 while substantiallyminimizing the overall signal and subsequent precision impact. In a 1Dcase, the pupil apodization profile may be selected according to thefollowing cost functional:

F(p(k))=∫_(x>x) ₀ dx[|∫ _(−NA) ^(+NA) dk p(k)e ^(−ikx)|²]+λ₁∫_(k<k) ₀dk|p(k)−p(0)|²,

where p(k) is a pupil apodizer profile, x₀ defines a target range of thesample, k₀ defines a target range of the pupil plane, λ₁ definesrelative weight for tail reduction in field plane and pupil functionuniformity, and NA defines pupil aperture (in natural units). Further,in embodiments, the collection apodization profile may be selectedaccording to the following cost functional:

F(p(x))=∫_(k>k) ₀ dk[|∫ _(−L/2) ^(+L/2) dx p(x)e ^(+ikx)|²]+λ₂∫_(x<x) ₁dx[|p(x)−p(0)|],

where p(x) is a field apodizer profile, x₁ defines a target range of thesample, k₀ defines a target range of the pupil plane, λ₂ definesrelative weight for tail reduction in field plane and collection fieldstop function uniformity, and L defines collection field stop size.

In some embodiments, the illumination spectrum can be varied as part ofthe metrology recipe setup. For example, the illumination spectrum maybe controlled utilizing a spectral controller or spatial light modulator(SLM), such as a DLP mico-mirror array manufactured by TEXASINSTRUMENTS. For example, apodization profile may be determined andcontrolled utilizing target related parameters such as size, pitch, orreflectivity as a function of wavelength together with similarcharacteristics of target proximity according to a cost functionalsimilar to the apodization profile selection cost functions describedabove.

Despite advantages of strong apodization functions, there may be anassociated loss of signal and subsequent loss of metrology precision. Insome embodiments, the shape of the pupil, hence the pupil function, maybe modified to regain metrology precision. FIGS. 10A and 10B illustratevarious pupil functions 200 that may be applied to the apodized pupil108. In some embodiments, as illustrated in FIG. 10A, the spots (pupilapertures) 202 a-202 d may be elongated in a direction orthogonal todiffraction in order to increase the size of the diffraction spot.Further, the apodized pupil 200 may be configured for quadrupoleillumination, including at least four elongated apertures 202 a-202 d,to ensure that the elongation is always in a direction orthogonal to thediffraction and to allow for capturing orders from higher ratios ofillumination wavelength to grating pitch. Further embodiments of thepupil 200 are illustrated in FIG. 10B. However, it is contemplated thatvarious modifications may be incorporated without departing from thescope of this disclosure.

While some of the embodiments discussed above are directed toapodization functions that are intensity modulated only, it isemphasized here that the apodization functions may be complex functionscombining intensity modulation together with phase apodization. Forinstance, the cost functions p(x) and p(k) given above for field andpupil apodization may be rewritten as p=|p|e^(iψ), where |p| reflectsintensity modulation of the apodizer and ψ phase modulations.

Apodization elements may be manufactured to several technologies knownto the art. Some examples include half-tone amplitude transmissionmasks, varying neutral density masks, and phase modulated masks.Lithographic techniques are known to work particularly well forhalf-tone amplitude masks and for phase masks with discrete phase steps(e.g. approximately 8 levels). In some embodiments, the apodizationelements may be made using standard e-beam writing techniques in resiston photomask blanks to produce the high precision apodization requiredfor the optical metrology system 100.

Although the embodiments discussed above and illustrated by the figuresshow a single optical column (i.e. single-line illumination path andsingle-line collection path), it will be appreciated by those skilled inthe art that multiple paths may exist in an optical column. For example,multi-path optical arrangements may be employed for differentillumination and collection polarization states, as described in US Pub.No. 2011000108892, incorporated by reference herein. In someembodiments, two or more polarization paths may be simultaneouslyapodized by a common apodizer or there may be separate apodizers foreach polarization path.

The combination of a scanning beam with apodization may providesignificant advantages. Scanning a spatially coherent beam allows theillumination spot size to be controlled for each target without the lossof light that changing the field stop size imposes. A system thatsupports spatially coherent illumination enables the smallest possiblespot on the target and subsequently the smallest possible target sizes.Furthermore, apodizing the pupil function (as opposed to the field)allows critical distribution to be kept stationary in the illuminationpupil during the spot scan. It is further noted that a scanning modulemay be utilized to induce intensity modulations in the illuminationbeamed from a source. Accordingly, the spot incident on a sample mayhave a wafer coordinate dependent overall intensity. This may allow foran effectively apodized illumination field stop for an incoherent lightsource. An important advantage of this combination is the improvedflexibility in selection of the apodization function. Further, anillumination field apodizer is introduced that does not inflictscattering side-effects due to its fabrication process.

Additional advantages of the foregoing embodiments include, but are notlimited to: reduction or elimination of signal contamination on thecollection pupil from scattered light from outside the targeted region;reduction or elimination of signal contamination on the collection pupilfrom scattered light from apertures along the illumination or collectionoptical paths; stable illumination pupil distribution during spot scan;reduced interaction between the spot and field stops and target edgeduring scan; and controlled scanning allowing tradeoff between peripheryinteraction or target edge diffraction and better target noiseaveraging.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. Program instructions implementing methods such as thosedescribed herein may be transmitted over or stored on carrier media. Acarrier medium may include a transmission medium such as a wire, cable,or wireless transmission link. The carrier medium may also include astorage medium such as a read-only memory, a random access memory, amagnetic or optical disk, or a magnetic tape.

All of the methods described herein may include storing results of oneor more steps of the method embodiments in a storage medium. The resultsmay include any of the results described herein and may be stored in anymanner known in the art. The storage medium may include any storagemedium described herein or any other suitable storage medium known inthe art. After the results have been stored, the results can be accessedin the storage medium and used by any of the method or systemembodiments described herein, formatted for display to a user, used byanother software module, method, or system, etc. Furthermore, theresults may be stored “permanently,” “semi-permanently,” temporarily, orfor some period of time. For example, the storage medium may be randomaccess memory (RAM), and the results may not necessarily persistindefinitely in the storage medium.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.

What is claimed is:
 1. A system for performing optical metrology,comprising: a stage configured to support a sample; at least oneillumination source configured to provide illumination along anillumination path; an apodizer disposed within a pupil plane of theillumination path, the apodizer configured to apodize illuminationdirected along the illumination path; an illumination scanner disposedalong the illumination path, the illumination scanner configured to scana surface of the sample with at least a portion of the apodizedillumination; an illumination field stop disposed along the illuminationpath, the illumination field stop configured to block a portion ofillumination directed along the illumination path from scanning thesurface of the sample; at least one detector configured to detect aportion of illumination scattered, reflected, or radiated from thesurface of the sample along a collection path; a collection field stopdisposed along the collection path, the collection field stop configuredto block a portion of illumination directed along the collection pathfrom being detected; and a computing system communicatively coupled tothe at least one detector, the computing system configured to determineat least one spatial attribute of the sample based upon the detectedportion of illumination.
 2. The system of claim 1, wherein theillumination scanner is disposed within the pupil plane of theillumination path.
 3. The system of claim 1, wherein the illuminationscanner is disposed along the illumination path between the apodizer andthe illumination field stop.
 4. The system of claim 3, wherein theapodizer is disposed between the at least one illumination source andthe illumination scanner.
 5. The system of claim 3, wherein theillumination field stop is disposed between the at least oneillumination source and the illumination scanner.
 6. The system of claim1, wherein the illumination field stop is disposed along theillumination path between the apodizer and the illumination scanner. 7.The system of claim 1, wherein the apodizer is disposed along theillumination path between the illumination scanner and the illuminationfield stop.
 8. The system of claim 1, wherein the illumination fieldstop comprises an apodized field stop.
 9. The system of claim 1, whereinthe illumination scanner is further configured to direct illuminationscattered, reflected, or radiated from the surface of the sample alongthe collection path to the at least one detector.
 10. The system ofclaim 1, wherein the at least one illumination source comprises acoherent illumination source.
 11. The system of claim 1, wherein anapodization profile of the apodizer is selected in accordance with thefollowing equation:F(p(k))=∫_(x>x) ₀ dx[|∫ _(−NA) ^(+NA) dk p(k)e ^(−ikx)|²]+λ₁∫_(k<k) ₀dk|p(k)−p(0)|², where F(p(k)) is a cost functional, p(k) is a pupilapodizer profile, x₀ defines a target range of the sample, k₀ defines atarget range of the pupil plane, λ₁ defines relative weight for tailreduction in field plane and pupil function uniformity, and NA definespupil aperture.
 12. The system of claim 1, wherein the collection fieldstop comprises an apodized field stop configured to apodize illuminationdirected along the collection path.
 13. The system of claim 12, whereinan apodization profile of the apodized collection field stop is selectedin accordance with the following equation:F(p(x))=∫_(k>k) ₀ dk[|∫ _(−L/2) ^(+L/2) dx p(x)e ^(−ikx)|²]+λ₂∫_(x<x) ₁dx[|p(x)−p(0)|], where F(p(x)) is a cost functional, p(x) is a fieldstop apodizer profile, x₀ defines a target range of the sample, k₀defines a target range of the pupil plane, λ₂ defines relative weightfor tail reduction in field plane and collection field stop functionuniformity, and L defines field stop size.
 14. The system of claim 1,further comprising: a spectral controller disposed along theillumination path, the spectral controller configured to affectapodization by controlling a spectrum of illumination directed along theillumination path.
 15. The system of claim 14, wherein the spectralcontroller includes a micro-mirror array, a plurality of activeshutters, or a selected filter.
 16. A system for performing opticalmetrology, comprising: a stage configured to support a sample; at leastone illumination source configured to provide illumination along anillumination path to illuminate a surface of the sample; an apodizedpupil disposed along the illumination path, the apodized pupil includingat least four elongated apertures configured to provide a quadrupoleillumination function, the apodized pupil further configured to apodizeillumination directed along the illumination path; an illumination fieldstop disposed along the illumination path, the illumination field stopconfigured to block a portion of illumination directed along theillumination path from impinging upon the surface of the sample; atleast one detector configured to detect a portion of illuminationscattered, reflected, or radiated from the surface of the sample along acollection path; and a computing system communicatively coupled to theat least one detector, the computing system configured to determine atleast one spatial attribute of the sample based upon the detectedportion of illumination.
 17. The system of claim 16, further comprising:an illumination scanner disposed along the illumination path, theillumination scanner configured to scan the surface of the sample withat least a portion of the apodized illumination.
 18. The system of claim17, wherein the illumination scanner is further configured to directillumination scattered, reflected, or radiated from the surface of thesample along the collection path to the at least one detector.
 19. Thesystem of claim 16, wherein the illumination field stop comprises anapodized field stop.
 20. The system of claim 16, wherein the at leastone illumination source comprises a coherent illumination source. 21.The system of claim 16, wherein an apodization profile of the apodizedpupil is selected in accordance with the following equation:F(p(k))=∫_(x>x) ₀ dx[|∫ _(−NA) ^(+NA) dk p(k)e ^(−ikx)|²]+λ₁∫_(k<k) ₀dk|p(k)−p(0)|², where F(p(k)) is a cost functional, p(k) is a pupilapodizer profile, x₀ defines a target range of the sample, k₀ defines atarget range of the pupil plane, λ₂ defines relative weight for tailreduction in field plane and pupil function uniformity, and NA definespupil aperture.
 22. The system of claim 16, further comprising: anapodized collection field stop disposed along the collection path, theapodized collection field stop configured apodize illumination directedalong the collection path, and further configured to block a portion ofillumination directed along the collection path from being detected. 23.The system of claim 22, wherein an apodization profile of the apodizedcollection field stop is selected in accordance with the followingequation:F(p(x))=∫_(k>k) ₀ dk[|∫ _(−L/2) ^(+L/2) dx p(x)e ^(−ikx)|²]+λ₂∫_(x<X) ₁dx[|p(x)−p(0)|], where F(p(x)) is a cost functional, p(x) is a fieldstop apodizer profile, x₀ defines a target range of the sample, k₀defines a target range of the pupil plane, λ₂ defines relative weightfor tail reduction in field plane and collection field stop functionuniformity, and L defines field stop size.
 24. A system for performingoptical metrology, comprising: a stage configured to support a sample;at least one illumination source configured to provide illuminationalong an illumination path to illuminate a surface of the sample; anapodizer disposed within a pupil plane of the illumination path, theapodizer configured to apodize illumination directed along theillumination path; an illumination field stop disposed along theillumination path, the illumination field stop configured to block aportion of illumination directed along the illumination path fromimpinging upon the surface of the sample; at least one detectorconfigured to detect a portion of illumination scattered, reflected, orradiated from the surface of the sample along a collection path; anapodized collection field stop disposed along the collection path, theapodized collection field stop configured apodize illumination directedalong the collection path, and further configured to block a portion ofillumination directed along the collection path from being detected; anda computing system communicatively coupled to the at least one detector,the computing system configured to determine at least one spatialattribute of the sample based upon the detected portion of illumination.25. The system of claim 24, further comprising: an illumination scannerdisposed along the illumination path, the illumination scannerconfigured to scan the surface of the sample with at least a portion ofthe apodized illumination.
 26. The system of claim 26, wherein theillumination scanner is further configured to direct illuminationscattered, reflected, or radiated from the surface of the sample alongthe collection path to the at least one detector.
 27. The system ofclaim 24, wherein the illumination field stop comprises an apodizedfield stop.
 28. The system of claim 24, wherein the at least oneillumination source comprises a coherent illumination source.
 29. Thesystem of claim 24, wherein an apodization profile of the apodizer isselected in accordance with the following equation:F(p(k))=∫_(x>x) ₀ dx[|∫ _(−NA) ^(+NA) dk p(k)e ^(−ikx)|²]+λ₁∫_(k<k) ₀dk|p(k)−p(0)|², where F(p(k)) is a cost functional, p(k) is a pupilapodizer profile, x₀ defines a target range of the sample, k₀ defines atarget range of the pupil plane, λ₁ defines relative weight for tailreduction in field plane and pupil function uniformity, and NA definespupil aperture.
 30. The system of claim 24, wherein an apodizationprofile of the apodized collection field stop is selected in accordancewith the following equation:F(p(x))=∫_(k>k) ₀ dk[|∫ _(−L/2) ^(+L/2) dx p(x)e ^(−ikx)|²]+λ₂∫_(x<x) ₁dx[|p(x)−p(0)|], where F(p(x)) is a cost functional, p(x) is a fieldstop apodizer profile, x₀ defines a target range of the sample, k₀defines a target range of the pupil plane, λ₂ defines relative weightfor tail reduction in field plane and collection field stop functionuniformity, and L defines field stop size.